Nanoconfined methane density over pressure and temperature: Wettability effect

Abstract

Clear knowledge about nanoconfined methane density is helpful for original gas-reserve evaluation, and production prediction in shale gas reservoirs. Although great efforts had been devoted to shedding light on this issue, the wettability effect, an impact induced by surface affinity towards methane molecules, has not received due attention. However, shale rock is composed of organic matter and inorganic matter, suggesting a wide range of surface affinity strength, as a result, figuring out the wettability effect on methane density inside nanopores is significantly necessary. In this article, the wettability effect is coupled with a previously established model for adsorption phase thickness, then the effective pore radius can be described as a function of surface contact angle, and primary pore size. After that, capturing the relative strength between fluid-fluid interaction and fluid-surface interaction, a robust model manifesting the shift of methane critical properties, as well as surface contact angle, is developed. Subsequently, a modified PR-EOS is utilized to calculate methane density, incorporating effective pore size and nanoconfined methane critical properties, both of which are correlated to the wettability effect. The model reliability is clarified with excellent agreements against methane density in bulk condition, and a total of 46 nanoconfined densities collected from previous contributions. Results show that (a) For a specified wettability effect, the shrinkage of pore size plays a detrimental role for the enhancement of methane density, its magnitude can reach 20%; (b) Weak surface affinity contributes to the increasing methane density, while the impact will be greatly mitigated in large pores; (c) Methane density in cylindrical nanopores is less than that in slit nanopores, stemming from the strong surface-fluid interactions in nanoscale cylindrical geometry. The article provides a practical yet reliable approach to evaluate nanoconfined methane density, expecting to enrich development theory for shale gas reservoirs.